1. Field of the Invention
The present invention relates to a novel carbazole derivative and a semiconductor nanocrystal containing this novel carbazole derivative bonded via a coordination bond or intermolecular force, where the semiconductor nanocrystal is effectively used for quantum dot-type organic EL elements with high light emission efficiency.
2. Description of the Related Art
In recent years, quantum dot-type organic EL elements have been proposed which use quantum dots of a semiconductor nanocrystal as a light emitting material. Nanoscaled small materials show intermediate behaviors between those as atoms or molecules and those as macroscopic solids (bulk form). Nanoscaled materials whose charged carriers and excitons are confined in all three-dimensional directions are called quantum dots, which increase in effective band gap with decreasing of their size. That is, when the size of quantum dots decreases, their absorption and light emission are shifted to the short wavelength side from the red region to the blue region. Also, by controlling quantum dots in composition and size in combination, it is possible to obtain a wide range of spectrum from the infrared region to the ultraviolet region. In addition, by controlling the distribution of sizes, it is possible to obtain a spectrum having a narrow half width and excellent color purity. By virtue of these characteristics, for example, there has been proposed a white light-emitting device having a light emitting layer containing several semiconductor nanocrystals (Japanese Patent Application Laid-Open (JP-A) No. 2009-527099). However, this proposal is unsatisfactory in light emission efficiency since the external quantum efficiency is about 0.3% to about 0.4%.
In order to efficiently emit light having desired color, there has been proposed a light emitting element containing: a first quantum dot monomolecular film located at the side of a hole transport layer; a second quantum dot monomolecular film located at the side of an electron transport layer; and an exciton forming layer located between the first and second monomolecular films (JP-A No. 2009-87754). However, this proposal does not specifically describe light emission efficiency or service life. There has also been proposed an element focusing on a protective material for quantum dots dispersed in a matrix material (JP-A No. 2009-99545). This proposal describes that considering a drop in light emission efficiency being caused by a capping agent (surfactant) present on the surfaces of quantum dots in the wet chemical process of producing quantum dots, an element exhibiting improved mobility of excitons to quantum dots and excellent light emission efficiency is provided by forming a state where a protective material is bonded via a coordination bond to the surfaces of quantum dots or a state where a protective material is present on the surfaces of quantum dots due to interaction (attractive force) between the surfaces of quantum dots and the protective material as well as satisfying a specific relationship among ionization potential (Ip), electron affinity (Ea) and band gap (Eg) of the protective material, Ip, Ea and Eg of the matrix material and Eg of the quantum dots. However, an element having satisfactorily high light emission efficiency cannot still be provided in this proposal.